The invention relates to the field of construction, taken in its broadest sense, and more specifically to the field of the building of various constructions, significant part of which is made of wood.
It relates more particularly to the production of large-sized structural elements such as slabs, suspended platforms or platforms mounted on wooden piles, capable of withstanding heavy loads which may be stationary (buildings for example), or moving, for example when the structure acts as a roadway for a civil engineering works vehicle or a truck.
While the present invention is particularly well suited to producing horizontal structures, it could equally well be used for producing vertical walls.
Furthermore, according to one form of embodiment, the invention is particularly well suited to producing constructions which, if necessary, can be dismantled, particularly in the context of temporary structures.
It has long been proposed for platforms mounted on wooden piles to be produced.
Referring to the appended FIG. 1, in order to produce such structures, metal section pieces (1) are placed on posts (2), themselves anchored in the ground.
Mounted at regular intervals along these metal section pieces (1) are wooden beams (3) on which bearing panels (4), generally made of wood or wood derivatives, are placed.
In such a structure, the load per unit area taken by each beam (3) corresponds to the load spread over the bearing panel (4) over a width equal to a spacing (e) or distance between beams.
Hence, in order to ensure sufficient load-bearing capability, it is necessary to choose beams (3) the dimensions of which are large enough to withstand the load which may either be stationary or possibly be moving.
By way of example, for a conventional beam with a span (L) of 4 meters, at a 1-meter spacing, loaded with 350 kg per square meter, to which is added the weight equivalent to the load transmitted by the wheel of a truck, or some other point load, namely 7500 kg placed at the middle of the span, it is necessary to use solid beams having a minimum section of 200 by 500 mm. Such a beam is therefore necessarily made of a glued-laminated structure which can therefore not be dismantled.
A first problem that the invention sets out to solve is that of producing structures which, if necessary, can easily be dismantled, for example in the context of temporary structures, so as to take account of ecological constraints demanding that the materials used in such a context be re-used.
Furthermore, one of the objectives of the invention is to make it possible to produce structures which may or may not be dismantled, at a lower cost than producing beams using the glued-laminated technique.
In consequence, one of the objectives of the invention is therefore to produce such structures using standard components, such as planks 200 mm wide (h), it being possible for the planks to be of various lengths and making it possible to produce structures, of a wide span (from 6 to 12 meters), able to withstand heavy loads, of long length, and to do so using elemental planks which may have a length of between just 2.5 and 5 meters, or for conventional industrial lengths of 2 and 4 or 5 meters.
In the field of building, in order to produce houses, for example, it has been proposed, as is apparent from DE-19537298, for wooden structures consisting of parallel planks to be produced so as, for example, to produce roofs or floors capable of taking very well distributed loads.
According to the teachings of that document, the parallel planks that make up the wooden structure are joined together by nailing, have their longitudinal axis parallel to the plane passing through the edges exterior to the said planks, these being offset alternately from one another perpendicularly to this plane.
While such a solution can be used to take well-distributed loads, it is not, on the other hand, suitable for withstanding high loads exerting stresses in localized regions.
Furthermore, it is totally precluded for such offset-plank structures to be used to produce constructions capable of withstanding moving loads, for example constructions acting as roadways, particularly for civil engineering works vehicles or delivery vehicles used for building buildings intended to be erected on a slab or platform made from wood.
Now, there has been found, and this is what forms the subject of the present application, a novel type of building slab intended to be mounted on a bearing structure and capable of withstanding a heavy, fixed or moving, load, comprising, in a way comparable of the teachings of the aforementioned DE 19537298, a base structure consisting of a number of parallel planks of rectangular cross section which are joined together by nailing or screwing.
In such a structure, the planks of rectangular cross section have their longitudinal axis parallel to the plane of the slab and are offset alternately from one another perpendicularly to the plane of the said slab.
The slab according to the invention is characterized in that the said base structure is covered, over at least the entire surface intended to take the load, with a continuous panel extending across its entire width, and which allows the loads exerted on the said panel to be spread across several lateral planks adjacent to those against which the load is directly exerted, this spreading being along an axis perpendicular to the said planks and therefore parallel to the direction in which the planks are screwed or nailed together.
According to one form of embodiment, a second panel may be arranged under the planks to further strengthen the slab.
In other words, the building slab according to the invention behaves like the combination of a beam consisting of the collection of planks, and of at least one upper panel to which the loads can be applied directly.
More specifically, the connection of planks nailed and/or screwed together makes it possible to obtain a beam of a width equal to its span or, in other words, a beam which extends across the entire width of the slab it supports.
The offset between two consecutive planks of this structure makes it possible to multiply the bending strength by a factor of 1.5 to 2 by comparison with a beam of a height equal to the width (h) of just one of the constituent planks and to make it far less deformable by reducing the slenderness ratio, which characteristic is the result of the ratio of the length of the span to the thickness of the slab.
In other words, the configuration given to the planks structure is particularly advantageous in terms of bending strength and in terms of deformability.
From another point of view, such a configuration makes it possible to obtain mechanical strength with a saving of material.
Furthermore, by virtue of the fact that the various planks are joined together by nailing or, preferably, by screwing, it may be possible for such a structure to be completely dismantled when no longer in use, this thus making it possible for the planks to be reused or recycled after dismantling.
In such a case, the load-spreading panel associated with the base structure will preferably consist of a wooden panel preferably of crossed microply type, has a strong axis in the direction of the fibres and a weak axis in the direction of two or three crossed plies, the said panel being screwed to the base structure with its strong axis perpendicular to the axis of the planks of the said structure.
When there is no desire to make disassembly easier, then according to an alternative form of the invention, the load-spreading panel may consist of a layer or sheet of concrete which may be poured into the base structure consisting of offset planks.
In such a case, metal rods may be incorporated into the layer of concrete in order to increase the shear strength and the quality of the composite wood-concrete section.
By virtue of the load-spreading panel that the slab according to the invention has, the load can be spread over several lateral consecutive planks adjacent to those against which the load is directly exerted, this spreading being along an axis perpendicular to the planks and therefore parallel to the direction in which the planks are screwed or nailed together.
Advantageously, the planks of the base structure are offset alternately by a distance of between half and two-thirds of their width (h), which, in terms of bending strength, means a 150 to 200% increase.
In practice, a sealing layer such as asphalt may possibly be poured onto the surface of the load-spreading panel.
When the load-spreading panel consists of poured concrete, this concrete enters the spaces formed between two offset planks at the same level, this improving the bond between the concrete and the wood, to give compound inertia with the concrete in compression and the wood in tension.
In other alternative forms of embodiment, it may be envisaged for the space between two offset planks at the same level to be filled with an acoustic or even alternatively a thermal insulant.
It is also possible to use the space between the planks for rooting trunking, ducting or else electric wires.
Furthermore, in order to meet transport constraints, it is possible to produce large-sized platforms using elemental slabs according to the invention, joined together and set in place on a bearing structure.
In this case, each slab has a cut-out capable of allowing it to be arranged with the beams of the bearing structure.
When several slabs are thus joined together, a seal is preferably produced at the joint between panels.
Furthermore, in order to improve the durability of the panel, chemical treatments may be applied to the surface.